Hi,
I use V 5.34.36
1.
To access points of the TGraph2D use:
Double_t * x = g2->GetX(); etc.
2.
Interpolate returns 0 for points on the edge of the TGraph2D.
For points inside it works if they coincide with points of the TGraph2D.
See macro below.
With the new triangulation software the point on edge does not return 0 (point (1,1) in your example).
With the master version of ROOT you macro gives me:
root [0]
Processing tg2.C...
Info in <TCanvas::MakeDefCanvas>: created default TCanvas with name c1
x[2] 3
3
2.5
2